This project is concerned with studies of the respiratory and cardiovascular systems of mammals and birds with primary emphasis on their performance during exercise and exposure to high altitude. The effects of exercise on oxygen consumption, as well as temperature regulation and water metabolism, are being studied in order to obtain basic information on the physiological effects of locomotion in higher vertebrates. For these purposes we find it useful to compare the performance of mammals with that of birds, the reason being that a better understanding of the respiratory system of birds may help explain why birds display a far better tolerance to low oxygen pressures (altitude) than do mammals. Therefore, the oxygen transport system and especially the role of blood flow is being examined. At the present time, particular emphasis is given to the supply of oxygen to brain and muscle.